How can you tell if a guy is hard??
either see it...it's usually hard (excuse the pun :)) to miss if he isn't wearing really loose clothes - though some loose clothes make it stand out even more.
or you can feel it..I.e. something "else" pressing against you if you are kissing him.
That is true, if you are good at reading body language though
you could feel it, or he has his hands in his pockets trying to cover it up
uhm it will get bigg,
hence the word hard.